1. Field of the Invention
This invention relates to an automatic focusing device for a camera or the like, and more particularly to a device for automatically focusing an image on a predetermined position by use of an optical refractive element stationarily provided in the optical system of the camera or the like.
2. Description of the Prior Art
There have been known various types of automatic focusing device for a camera or the like. The conventional automatic focusing device employs a photodetector located in a plane on which an image should be focused by an objective and moves the objective back and forth along its optical axis in accordance with an outputted of the photodetector. The output of the photodetector shows a maximum value when the image is focused precisely on thereon.
The conventional automatic focusing device as described above is disadvantageous in that the change in output of the photodetector is slow and the focusing operation is accordingly slow. Particularly because the focus detection is conducted by detecting a maximum output, the objective must be moved back and forth to find the maximum value. In other words, it is impossible or very difficult to know if the image is focused at any position on the optical axis of the objective.